1. Field of the Invention
This invention relates to a substrate height position detecting apparatus, and particularly to a substrate height position detecting apparatus for setting the surface of a substrate such as a wafer at the focus position of a projection optical system in an exposure apparatus.
2. Related Background Art
The technique disclosed in U.S. Pat. No. 4,558,949, etc. is known as a technique of detecting the height position of a substrate of this kind. This is a technique of projecting a beam of imaging light from an oblique direction to the surface of a substrate (wafer), receiving the reflected beam of light from the wafer by a photoelectric detector through a slit plate while vibrating the reflected beam of light, synchronously detecting and processing a photoelectric signal outputted from the photoelectric detector by a vibration signal for vibrating the reflected beam of light, finding the amount of difference between the imaging plane (best focus plane) of a projection optical system and the surface of the wafer in the direction of height (the direction of the Z-axis), servo-controlling a Z stage so that said amount of difference may become 0, and bringing the height position of the wafer into coincidence with the best focus plane.
Also, in recent years, a height position detecting system (hereinafter referred to as the "multipoint AF system") for detecting the height positions at a plurality of points on a wafer has been proposed because if the measuring point for the measurement of the height position of the wafer is single, an accurate height position cannot be detected under the influence of the level difference of the surface of the wafer. This multipoint AF system applies, for example, a plurality of slit lights arranged at a predetermined pitch from an oblique direction to the wafer, and highly accurately detects the height positions at a plurality of points on the wafer on the basis of the relative position between the reflected lights of the plurality of slit lights from the wafer and a plurality of light receivers (light receiving elements) corresponding to the respective slit lights.
Also, Japanese Patent Application Laid-Open No. 63-161616 discloses, in a height position detecting system for projecting a beam of imaging light onto a point on a wafer and also receiving the reflected light thereof by a light receiving element, thereby detecting the height position of a wafer, a technique whereby when the reflected light from the wafer deviates from the light receiving area of the light receiving element (when the height position of the wafer deviates from the range of height position detectable by the light receiving element), that reflected light is photoelectrically detected by sensors for detecting the direction of deviation provided on both sides of the light receiving element, thereby detecting the direction of deviation of the wafer from the best focus position.
However, in the multipoint AF system as described above, the provision of sensors for detecting the direction of deviation on both sides of a plurality of light receiving elements as in the prior art has made the apparatus construction of the multipoint AF system bulky (limits the arrangement of the measuring points and the optical system) and has not been realistic. Particularly, in the multipoint AF system wherein a plurality of slit lights are applied from an oblique direction to a wafer to thereby form a plurality of slit images arranged at a predetermined pitch in the direction of measurement on the wafer, and the reflected lights of the respective slit lights are individually detected by a plurality of light receivers, the slit images on the wafer deviate in the direction of arrangement of the slit images by the height position of the wafer deviating from a target position. When the amount of deviation between these slit images on the wafer becomes 1/2 of the pitch of the slit images (the interval between two adjacent slit images), these reflected lights enter the corresponding light receivers while deviating from one another. Accordingly, the other light receivers than the endmost light receiver output signals similar to those when the surface of the wafer is at the target position. In spite of the surface of the wafer deviating from the target position at this time, there has been a problem that the multipoint AF system erroneously detects the then position of the surface of the wafer as the target position.